An organic compound (organic molecule) gets to have energy (excited state) by absorbing light. Through the excited state, various reactions (photochemical reactions) and luminescence may be generated, and are used for various applications.
As an example of photochemical reactions, there is a reaction of a singlet oxygen with an unsaturated organic molecular (oxygenation) (for example, refer to Non-Patent Reference 1). Oxygen in a singlet state (singlet oxygen) is not be generated by direct photoexcitation since the ground state of an oxygen molecule is a triplet excited state. However, in the presence of other triplet excited molecules, singlet oxygen is generated to enable an oxygenation reaction. In this case, a compound capable of forming the triplet excited molecules is referred to as a photosensitizer.
As mentioned above, a photosensitizer capable of forming triplet excited molecules by photoexcitation is necessary for generating singlet oxygen. However, the ground state of an organic compound is normally a singlet ground state. Thus, a photoexcitation to a triplet excited state is a forbidden transition and a triplet excited molecular is unlikely to be generated (a singlet excited molecular is normally generated). Therefore, for such a photosensitizer, a compound in which intersystem crossing from a singlet excitation state to a triplet excitation state tends to occur (alternatively, a compound which allows a forbidden transition of photoexcitation directly to a triplet excited state) is required. That is to say, it is possible and effective to use such a compound a photosensitizer.
In addition, such a compound can often emit phosphorescence. Phosphorescence is luminescence generated by the transition between energy states that are different in multiplicity, and in the case of a common organic compound, indicates luminescence generated in returning from a triplet excited state to a singlet ground state (on the other hand, luminescence generated in returning from a singlet excited state returns to a singlet ground state is referred to as fluorescence). Application fields of a compound capable of emitting luminescence, that is, a compound capable of converting a triplet excited state into luminescence (hereinafter, referred to as “phosphorescent compound”) includes a light-emitting element using an organic compound as a luminescent compound.
The light-emitting element has characteristics such as slimness and lightweight, high-speed response, direct-current low-voltage driving. Therefore, the light-emitting element is a device attracting attention as the next-generation flat-panel display element. In addition, since the visibility is relatively favorable due to light emission by itself and a wide viewing angle, the light-emitting element is considered to be effective as element to be used for a display screen of a portable device.
In the case of using an organic compound as a light emitter, the emission mechanism of the light-emitting element is included a carrier-injection type. Namely, when a voltage is applied to electrodes with a light-emitting layer interposed therebetween, an electron injected from a cathode and a hole injected from an anode are recombined in the light-emitting layer to form an excited molecule, and energy is released to emit light when the excited molecule returns to the ground state.
In addition, as the type of the excited molecule, an excited singlet state (S*) and an excited triplet state (T*) are possible as in the case of the above-mentioned photoexcitation. In addition, it is believed that the statistical generation ratio in the case of the light emitting element is S*:T*=1:3 (for example, refer to Non-Patent Reference 2).
However, in the case of a common organic material, luminescence (phosphorescence) from a triplet excited state is not observed at room temperature, and normally, only luminescence (fluorescence) from a singlet excited state is observed. This is because the ground state of an organic compound is normally a singlet ground state (S0), and thus, T*→S0 transition (phosphorescence process) is a strongly forbidden transition and S*→S0 transition (fluorescence process) is an allowed transition.
Accordingly, in the case of the light-emitting element, the theoretical limit of the internal quantum efficiency (the ratio of generated photons to injected carriers) is considered to be 25% on the ground of S*:T*=1:3.
However, T*→S0 transition (phosphorescence process) is allowed when the phosphorescent compound is used, and thus, the internal quantum efficiency can be 75% to 100% theoretically. Namely, the luminous efficiency can be 3 to 4 times as high as a conventional luminous efficiency. In fact, light-emitting elements using phosphorescence compounds have been released one after another, and the luminous efficiency has been attracting attention (for example, refer to Non-Patent Reference 3 and Non-Patent Reference 4).
In Non-Patent Reference 3, a porphyrin complex with platinum as a central metal is used, and in Non-Patent Reference 4, an organometallic complex with iridium as a central metal is used. The complexes are both phosphorescent compounds.
In addition, by alternately stacking a layer including an organometallic complex with iridium as a central material (hereinafter, referred to as “iridium complex”) and a layer including DCM2 that is a known fluorescent compound, it is possible that triplet excitation energy generated in the iridium complex is transferred to DCM2 to contribute to the luminescence of DCM2 (for example, refer to Non-Patent Reference 5). In this case, since the amount of singlet excited state of DCM2 (normally, 25% or less) is amplified more than usual, the luminous efficiency of DCM2 is increased. This can be said to be also sensitization of the iridium complex, which is a phosphorescent compound.
As shown in Non-Patent Reference 3 to Non-Patent Reference 5, a light-emitting element using a phosphorescent compound can achieve a higher luminous efficiency than ever before (namely, less current makes it possible to achieve a higher luminous efficiency). Therefore, it is considered that the light-emitting element using the phosphorescent compound will give greater importance in the future development as a method for achieving luminescence with a higher luminance and a high luminous efficiency.
As described above, a phosphorescent compound tends to be occurred intersystem crossing and to generate luminescence (phosphorescence) from a triplet excited state. Therefore, the phosphorescent compound is an expected compound since the phosphorescent compound is useful for using as a photosensitizer and for applying to a light-emitting element as a phosphorescent material. However, the current state is that the number of photophorescent compounds is small.
As one of the few the phosphorescent compounds, the iridium complex used in Non-Patent Reference 4 or Non-Patent Reference 5 is one of organometallic complexes referred to as an orthometalated complex. The complex has a lifetime of several hundreds nanoseconds, and a high phosphorescent quantum yield. Therefore, since the decrease in efficiency due to increase in luminance is small as compared with the above-mentioned porphyrin complex, the complex is effective in a light-emitting element. Also in that way, such an organometallic complex is one of guidances for synthesizing a compound in which direct photoexcitation to a triplet excited state and intersystem crossing tend to occur, consequently a phosphorescent compound.
The structure of a ligand of the iridium complex used in Non-Patent Reference 4 or Non-Patent Reference 5 is relatively simple and shows green luminescence with favorable color purity. However, the structure of the ligand needs to be changed to change the luminescent color to other colors. For example, in Non-Patent Reference 6, various ligands and iridium complexes using the ligands are synthesized, and some luminescent colors are realized.
However, many of the ligands have difficulty in being synthesized or have many steps required for synthesizing, which leads to price increases of materials themselves. In these organometallic complexes, though it is often the case that iridium or platinum is used as a central metal to emit phosphorescence, these metals themselves are expensive, and additionally, the ligands also become expensive. In addition, blue luminescence with favorable color purity has not been realized.
Further, in Non-Patent reference 7, an iridium complex using dibenzo[f,h]quinoxialine derivative as a ligand is synthesized. A light-emitting element using those shows orange-red luminescence with a high efficiency. Red luminescence with favorable color purity has not been realized.
In addition, an organometallic complex is easily decomposed commonly. Even in the case of an organometallic complex which is awkward to be decomposed, the thermal decomposition temperature thereof is never high. Namely, an organometallic complex is poor in heat resistance, which becomes problem in applying to an electronic device as a light-emitting element.
The descriptions above show the necessity to synthesize an organometallic complex that is excellent in also heat resistance with the use of a ligand which is capable of being synthesized easily and changing a luminescent color to other colors. This is because inexpensive and various photosensitizers and phosphorescent materials (that is, materials in which intersystem crossing to a triplet excited state tends to occur) can be obtained.    Non-Patent Reference 1: Haruo INOUE, and three others, Basic Chemistry Course PHOTOCHEMISTRY I (Maruzen Co., Ltd.), 106-11    Non-Patent Reference 2: Tetsuo TSUTSUI, Textbook for the 3rd Workshop, Division of Molecular Electronics and Bioelectronics, Japan Society of Applied Physics, 31 (1993)    Non-Patent Reference 3: D. F. O'Brien, and three others, Applied Physics Letters, vol. 74, No. 3, 442-444 (1999)    Non-Patent Reference 4: Tetsuo TSUTSUI, and eight others, Japanese Journal of Applied Physics, vol. 38, L1502-L1504 (1999)    Non-Patent reference 5: M. A. Baldo, and two others, Nature (London), vol. 403, 750-753 (2000)    Non-Patent Reference 6: Mark E. Thompson, and ten others, The 10th International workshop on Inorganic and Organic Electroluminescence (EL'00), 35-38    Non-Patent Reference 7: J. Duan, and two others, Advanced Materials (2003), 15, No. 3, February 5